1. Field of the Invention
This invention relates to test equipment and more particularly to equipment for testing an interrogator and transponder used for identification friend or foe on an aircraft.
2. Description of the Prior Art
Presently, the identification of aircraft flying in a region as friendly or unfriendly is provided by identification friend or foe (IFF) equipment. A ground station or an aircraft may send out an interrogation pulse which is received by a transponder in the interrogated aircraft which generates a reply signal in response to the interrogation pulse. The interrogation pulse and the reply pulse may be in the form of messages which contain certain information which may be enciphered in the form of a cryptogram. The cryptogram may be generated by a cryptograph coupled to the interrogator and by a cryptograph coupled to the transponder. The round trip time delay from the transmission of the interrogation pulse to the reception of the reply pulse is an indication of the distance to the interrogated aircraft. Typically, each aircraft carries an interrogator and a transponder as part of its identification friend or foe equipment. A cryptograph may be coupled to the interrogator when the use of coded messages or cryptograms is desired. Likewise, a cryptograph is coupled to the transponder for receiving coded messages or cryptograms and for generating reply messages or cryptograms. The interrogation pulse may be sent out on a first frequency, for example 1030 MHz and the reply pulse from the interrogated aircraft may be at a second frequency, for example, 1090 MHz. Typically, the interrogation pulse and reply pulse may be a train of subpulses with individual subpulses having a duration from 0.45 to 1.0 us. The interrogation pulse and/or the reply pulse may have a duration in the range from 3 to 150 us.
The interrogator and transponder as part of the IFF equipment on an aircraft may have separate antennas. The interrogator has a receiver for receiving reply messages and a transmitter for transmitting interrogation messages. The transponder has a receiver for receiving interrogation messages and a transmitter for transmitting reply messages. In order to prevent the aircraft's transponder from receiving, processing and replying to its own interrogator which would interfere with the IFF process, the interrogator provides a suppression signal to the aircraft's transponder. Likewise, when the transponder is transmitting the transponder provides a suppression signal to the interrogator to prevent the interrogator from receiving and processing the aircraft's replies. Thus, when the interrogator is transmitting the transponder is turned-off and when the transponder is transmitting the interrogator is turned-off.
Prior to take-off, an aircraft containing identification friend or foe equipment is tested by testing the interrogator and transponder separately. A transponder would be used to test the interrogator and an interrogator would be used to test the transponder. If cryptograms are used then the test equipment would need a cryptograph for the interrogator and one for the transponder. Alternatively, the test equipment could store predetermined cryptograms which would be valid at the time of testing of the identification friend or foe equipment.
It is therefore desirable to provide test equipment containing a single transmitter and receiver for testing both the interrogator and transponder of IFF equipment on an aircraft.
It is further desirable to provide a method of testing IFF equipment using cryptograms with test equipment that does not include a cryptograph or stored cryptograms.
It is further desirable to test IFF equipment on an aircraft prior to take-off with test equipment in close proximity which can handle cryptograms without needing a cryptograph.